It's passive aggressive because it's a passive statement that carries a strong threat of negative reaction if it isn't followed.
Something like "will you please stop kicking the seat" carries a strong threat relative to which other variant? It's more passive than which other variant? Or do you claim that it's a passive voice grammar-wise? (It isn't.)
And there's nothing wrong with being irritated.
The phrase used to mean responses like "I wonder why some people act like moronic pieces of shit about others' seats". See how it's non-direct but aggressive at the same time?